the Diffusion of Gases. 231 



As soon as it was filled, it was placed in a glass-jar, of about 

 the same height, with a little water left in the bottom, and in 

 proportion as the water rose in the tube of A, from the subse- 

 quent contraction, the jar was filled up by repeated additions of 

 water, so as to keep the surface of the water, within and without 

 the tube, as nearly as possible at the same level. With the view 

 of having the external air in a constant state in regard to humi- 

 dity, means were taken to saturate it. A small cone of damp 

 paper was inverted, like an extinguisher, over the upper part of 

 the instrument ; the jar containing the instrument was placed on 

 the shelf of the pneumatic trough, and a bell-jar with an opening 

 at the top, which could be shut at pleasure, inverted over the 

 whole. The return-air must therefore have been in the same state, 

 in regard to humidity, as the hydrogen itself. Aqueous vapour 

 would diffuse neither outwards nor inwards, as it existed in the 

 same proportion on both sides of the plug ; but dry hydrogen 

 only would be exchanged for dry air, in the proportion of their 

 equivalent diffusion-volumes. 



In the first thirty-four minutes, the gaseous contents of the 

 bulb were diminished by 95 measures, and ultimately, in twenty- 

 six and a half hours, they were reduced to 227 measures, which 

 were common air. The contraction in this and other cases, in 

 which the water rose into the bulb, was determined by weigh- 

 ing, at the end of the experiment, the water which had entered ; 

 a mode which admits of even greater nicety than measuring the 

 bulk of residuary gas in a graduated vessel. 



With the view of obtaining elements for a correction for any 

 change in the bulk of the gas, which might take place during 

 the continuance of the experiment, from changes 

 in temperature, pressure, or from solution of the ( 

 gas in water, a receiver was made of the same 



tube, with a bulb of nearly the same capacity as 



6 



the diffusion instrument, but close at the top. 



Gg 2 



